Polyethylenes having broad MWD are widely used in the art in various applications, such as fibers, moldings, films and pipes, in view of the improved properties that they exhibit over narrow molecular weight distribution polyethylenes. Such polyethylenes may be multimodal, i.e. may present molecular weight distribution curves having more than one molecular weight peak, due to the presence of a plurality of polymer fractions having different molecular weights. In view of their broad MWD, this kind of polyethylenes can also be processed more easily with respect to monomodal, narrow MWD polyethylenes.
Various methods are known in the art to produce polyethylenes with broad MWD, including post reactor melt blending, single stage processes carried out in the presence of a catalyst system comprising a mixture of different catalysts, and multistage processes. The method used determines the properties of the polyethylene, in that the properties of the obtained polyethylene depend not only on the properties of the single polymer fractions thereof, but also by the quality of the mixing of these fractions.
Various families of polymerization catalysts are known in the art, such as Ziegler catalysts, metallocene catalysts and the more recently developed transition metal complexes comprising ligands other than cyclopentadienyl. For instance, WO 98/27124 discloses 2,6-bis(imino)pyridyl complexes of iron and cobalt as catalysts for homo- or co-polymerization of ethylene. J. Am. Chem. Soc. 127, 13019-13029 (2005) describes the preparation of several bis-iminopyridinato Ziegler catalysts and their activities in the polymerization of ethylene.
Polyethylene having broad MWD may be produced in a single reactor by using catalyst systems comprising at least two different ethylene polymerization catalysts, able to produce polyethylene fractions having different molecular weight. For instance, WO 95/11264 discloses the use of a combination of catalyst compositions comprising at least two different ethylene polymerization catalysts of the Ziegler type or the metallocene type to produce a polyethylene having a broad molecular weight distribution.
WO 99/46302 describes a catalyst composition for the polymerization of alpha-olefins comprising a 2,6-bis(imino)pyridyl iron catalyst and another catalyst, such as a zirconocene or a Ziegler catalyst. These mixed catalyst systems are able to give, in a single polymerization step, polyethyelene products having broad MWD and short-chain branches, like C2 and C4 branches, but the products normally do not show the presence of long-chain branching. In fact, the 2,6-bis(imino)pyridyl iron catalysts are known to produce linear polyethylene, as indicated for instance in WO 2007/111499, where the iron catalyst is used in combination with different catalysts components (like Ni, Cr and Pd catalysts), which are able to produce polyethylene with short branching, in order to prepare substantially linear polyethylene products.
In is known in the art that branched PE products may be obtained by the in situ polymerization of ethylene with combined iron catalyst and traditional Ziegler-Natta catalyst. The mixed catalyst system is commonly used in a tandem reaction, where higher 1-alkene is produced from ethylene oligomerization by one catalysts (normally the iron catalyst), and it is then copolymerized by the other catalyst (Ziegler-Natta catalyst) in situ with ethylene to produce linear low density polyethylene (LLDPE). Nevertheless, the obtained products mainly contain short branches, such as ethyl and butyl, and only minor amounts of higher branches (such as hexyl and longer than hexyl).
An approach investigated in the past to expand the branching level of polyethylene was to incorporate higher comonomers; nevertheless, this gives problem of different reactivity of the catalysts with respect to the different comonomers, thus requiring a high outlay in terms of apparatus; moreover, the obtained branches are still short.
Therefore, it is felt the need for a process able to provide, in acceptable yields, ethylene polymers having broad MWD and high amounts of long-chain branches.